1. Field of the Invention
The field of invention relates to the production of subsurface crude oil deposits. More specifically, the field relates to a system and method for using downhole oil/water separation (DOWS) system for improving injectivity and use of formation water and for improving hydrocarbon recovery from a plurality of hydrocarbon-bearing formations.
2. Description of the Related Art
Downhole Oil/Water Separation (DOWS) systems that produce hydrocarbon fluids (crude oil, natural gas condensates) have been used in the field for nearly 20 years. Oil and gas industry engineers developed various technologies that separate hydrocarbons from water inside a well. If the entire process of lifting, treating, and reinjecting produced water can be avoided, costs and environmental impacts are likely to be reduced. The idea is that a hydrocarbon-rich fluid is produced to the surface while a water-rich stream is redirected and injected into an underground formation, for example, a second well leg or a non-hydrocarbon-bearing and porous part of the reservoir, without being lifted to the surface.
Management of produced water presents challenges and costs to operators and to the environment. Produced water is underground formation water that is brought to the surface along with crude oil, natural gas condensate, or natural gas. Typically, it is the largest (in volume) by-product or waste (depending how and if it is used in other processes) associated with oil and gas production. DOWS systems are intended to reduce the amount of water produced to surface in order to minimize the surface facility requirements for separating and treating water. Produced water separation, treatment and disposal per barrel of hydrocarbon fluid can be expensive given the amount of chemicals and additives required in addition to the on-site infrastructure—temporary or permanent—needed to process the water-laden hydrocarbon. The cost of managing produced water after it is already lifted to the surface and separated from the oil or gas product can range from less than $0.01 to more than several dollars per barrel. With a number of DOWS systems, production water can be reduced by up to 75% of the production water potentially produced to the surface without such a system in place.
Two basic types of DOWS systems are currently in use. One type of system uses a hydrocyclone to mechanically separate oil from water and direct the two products away from one another. The other type of DOWS system relies on gravity separation that takes place in the well bore. There is also a third type that potentially could be used—a membrane separation-based system—but there is not wide acceptance of this type of system given that the other two are well-established and proven technologies with known economic feasibility.
DOWS systems envision the use a single well in which an oil/water mixture was lifted to the DOWS system, fluid components separated out via either a hydrocyclone or gravity separation, and the water injected into a “suitable” formation that is different and not in fluid communication with the producing hydrocarbon-bearing formation except via the well.
DOWS systems, however, suffer from several systemic problems. Many DOWS systems are abandoned or are no longer performing to their full potential due to various factors. One reason may include plugging or low permeability at the injection zone for the formation that is to receive the production water (fines including sands, insoluble salts, minerals and clays; hydrocarbon residuum). The injection formation is typically in fluid contact with the well containing the DOWS system and is often a water-bearing formation or is a naturally permeable formation, such as sandstone. Another reason is having a high water cut, where the water fraction overwhelms the ability of the DOWS system to separate the hydrocarbons from the water effectively. Poor injection formation fluid isolation from the hydrocarbon-bearing formation can be fatal to a DOWS system. If isolation is not sufficient, the injectate (produced water) can migrate into the producing zone, thereby short-circuiting the produced hydrocarbon and formation water fluid flow pathway and re-entering the production perforations. This results in produced water recycle, and hydrocarbon production dropping to nearly zero. Corrosion and scale in the well bore annulus or casing surface can also be a major factor as this may clog the produced water flow pathway.